The invention relates to a humidifying solution for offset print sheet and to a procedure and to a device for the preparation of said humidifying solution.
Specifically, the humidifying solution of the invention is of the type that are formed by between 95% and 97% of water and between 3% and 5% of an additive which contains sodium citrate, acetic acid, citric acid and sodium benzoate.
The procedure to obtain the humidifying solution to which the invention refers is of the type which contains a preliminary phase of oxygen adsorption by the means of projecting a thrust, over the free surface of the humidifying solution, of recircled fractions of the humidifying solution.
The device for the preparation of the humidifying solution according to the invention is of the type which contains a main container reservoir for the solution, an outlet pipe for the solution provided with a first pump that recircles part of the solution back to some means for thrust propulsion of the recircled fraction over the free surface of the solution, which is present in an annex container reservoir, main and independent from it, a feeding pipe which communicates said annex compartment with the humidifying bucket for the humidifying cylinder of an offset printing machine and a feedback pipe for the solution in the humidifying bucket to main container reservoir.
The offset printing is an indirect printing fashion in which three cylinders are used: the printing sheet is mounted on the first cylinder; the second cylinder transmits the image to a second cylinder, which is covered by a rubber layer, which prints the paper which traverses between itself and a third cylinder which is only for pressing purposes. The pressing sheet is covered with photo-sensible emulsions in the shape of colloids which, when exposed to light, polymerize creating some surface zones which are lipophilic or similar to printing ink, which are fat bodies and hidrophilic metallic zones without images, which repel the ink.
A first set or rollers provide the printing ink to the first cylinder that the sheet has and the other set of rollers wets the sheet. The image zones accept ink, while the blank ones reject it. The addition of wetness to the sheet is achieved using what are called wetting systems.
The offset printing is a system based in surface physic and chemical phenomena, that is it is not a system based on relief, like typography, flexography, tampography, etc., but it is a planographic system in which the phase previous to the transferring of the ink to the paper consists in chemical attractions and repulsions between the different lipophilic and hidrophilic substances. Therefore, both the water and the ink must have a set of adequate physical and chemical characteristics that allow that the process be the right one.
Around this idea, and keeping in mind that the water with which the offset machines work is regular network water, there is the need to add some special additives to the water so that it gets determined properties in pH and surface tension, among others.
Among the wetting systems, the water alcohol system can be pointed out. It consists in adding, besides some additives, between 10% and 12% of isopropil alcohol to the water. The water alcohol system is the most developed and extended one because the quality parameters with this system are far better than with any other system, for example those called in the xe2x80x9cintegralxe2x80x9d and xe2x80x9cconventionalxe2x80x9d techniques. The basis for this good behavior is that the isopropyl alcohol provides a good interfacial tension between the water and the ink which brings the consequence a better water in the ink emulsion, so the printed ink is more vivid than with any other known wetting system.
Some other additional advantages are that the wetting system with isopropylic alcohol provides a higher viscosity that allows a more uniform image transference, a faster drying speed and a decrease in the surface tension that allows the water layer over the sheet to be very thin and to have a lower tendency to the mechanical emulsion.
However, for some years now, very rigorous studies are being conducted on the possible toxicity that the isopropylic alcohol based systems can present due to a prolonged exposure. In this sense, some countries have absolutely forbidden its use, while in some others its use has been largely restricted and in our country it is on its way to being forbidden for good.
Apart from toxicity, they present storing difficulty problems and danger of inflammability.
Many manufacturers have unsuccessfully tried to formulate additives that satisfy the aforementioned quality needs and refrains completely from using the isopropyl alcohol. The reason of the failed tries is in essence that the need to have to reduce the surface tension to avoid that the water ink emulsion to levels which can be compared to the water alcohol system obligates the use of tensoactives of group III. A larger concentration of these tensoactives originates a decrease in the interphase by affinity between the ink fat acids and the tensoactives hydro-carburated strings. The tensoactives cause a great deal of foam and, because they are no volatile, they remain in the ink rollers, thus accumulating and causing operating problems.
The former, together with the impossibility to form stable colloids based on other substances that reduce the surface tension and maintain a larger water ink interphase are the main reasons why it has been impossible to substitute the isopropyl alcohol.
European patent number EP-A-0 770 501 introduces some improvements in wetting systems for offset printing, consisting in feeding the wetting system with a dissolution based on water and some determined additive. The dissolution is subjected to some collision phenomena during which it acquires, due to an adsorption process, an important amount of oxygen, with which it is achieved that the dissolution that wets the printing sheet, based on which the impression is made, acquires a large amount of free oxygen, thus increasing the water ink inter-phase.
Also, the EP-A-0 770 501 describes the device for the preparation of the solution, which consists of a main container reservoir for the solution, an outlet pipe provided with a first pump that recircles part of the solution towards some means for thrust projection of the recircled fraction over the free surface of the present solution in a compartment annex to the main container and independent from it, a feeding pipe, which communicates said annex compartment with the wetting bucket of the wetting cylinder of an offset printing machine and a feedback pipe for the wetting bucket solution to the main container reservoir.
Said improvements and device, even if they have a correct functionality and provide a good final quality, lack that the collision of the recircled fraction against the free surface does not achieve a constant adsorption isothermal on the surface of the solution, which would be desirable for getting a larger uniformity of extension of the humidifying layers over the printing sheet.
The goal of the invention is to provide an effective solution to all problems listed above.
To that extent, the invention makes public the humidifying solution, which in essence is characterized because the aforementioned additive contains between 0.0% and 0.5% in weight of sodium hexametaphosphate, between 0.0% and 1.5% in weight of ascorbic acid, between 0.0% and 1.0% in weight of sodium benzoate, between 0.0% and 1.0% in weight of citric acid, between 2% and 10% in weight of glycerin, between 0.0% and 0.1% in weight of limonal, between 50% and 75% in weight of sorbitol, between 1% and 2% in weight of acetic acid, between 3% and 7% in weight of citric acid, between 0.0 and 1.0% in weight of sodium oleate and between 15% and 30% in weight of osmotic water.
According to one other feature of the humidifying solution of the invention, the aforementioned additive is constituted by 0.1% in weight of sodium hexametaphosphate, 0.3% in weight of ascorbic acid, 0.2% in weight of preventol, 0.2% in weight of sodium benzoate, 0.2% in weight of citric acid, 5% in weight of glycerin, 0.05% in weight of limonal, 65% in weight of sorbitol, 1.5% in weight of acetic acid, 5% in weight of sodium citrate, 0.2% in weight of sodium oleate, and 22.25% in weight of osmotic water.
The invention also has the goal to a method to obtain the aforementioned humidifying solution, characterized because the solution is forced to flow at high speed by a pipe with a helicoidal configuration and is projected thus forming a 90-degree solid angle output cone, in such a way that a part of the particles of the projected thrust collide against the free surface of the humidifying solution, obtaining on said free surface a isothermal of constant adsorption.
According to one other feature of the invention, the humidifying solution is projected at a lineal speed between 1,3 m/s and 220 m/s and at a pressure up to 1,500 kg/cm2.
According to one other feature of the invention, said lineal speed is 60 m/s and said pressure is 250 kg/cm2.
Another goal of the invention is a device for the preparation of a humidifying solution according to the invention using the method described above, which in essence is characterized because the aforementioned projection means are constituted by an injector which makes the solution turn at high angular speeds in a helicoidal trajectory and subject to very high pressures, so that the solution is projected forming a output cone of 90 degrees of solid angle, so that a part of the particles of the projected thrust collide against a deflecting surface and the rest of the particles collide against a free surface of the humidifying solution an isothermal of constant adsorption.
Preferably, the aforementioned injector has an initial acceleration part in a helicoidal trajectory and right after it a second diffusion part of toroidal configuration, so that the humidifying solution is projected over said free surface and against said deflecting surface at a lineal speed between 1,3 m/s and 220 m/s and at a pressure up to 1,500 kg/cm2.
In an actual realization of the invention, the lineal speed is 60 m/s and said pressure is 250 kg/cm2.
According to one other feature of the invention, said deflecting surface makes an angle of between 0 degrees and 30 degrees with said liquid free surface.
Preferably, said deflecting surface (4) makes a 15-degree angle with the said liquid free surface.